Insulated cathode gun device

ABSTRACT

A cathode gun device having an improved heat shield for greater operating efficiency is disclosed. The device includes a tubular cathode electrode, a heater electrode, a plurality of heat shields that are heat reflective, and a plurality of heat shields that are of low thermal conductivity arranged in an alternate fashion.

finite States atent n91 Basiulis I INSULATED CATHODE GUN DEVICE [75] Inventor: Algerd Basiulis, Redondo Beach,

Calif.

[73] Assignee: Hughes Aircraft Company, Culver City, Calif.

[22] Filed: Apr. 9, 1973 [21] Appl. No.: 349,157

313/346, 315/35 [51] Int. Cl H01j 1/00, 1 101k 1/02 [58] Field of Search 313/326, 239, 240, 242,

[56] References Cited UNITED STATES PATENTS A 2,518,879 8/1950 Gerrnesha'usen 3'13'/240 x May 28, 1974 2,888,592 5/1959 Lafferty 313/356 X 2,899,591 8/1959 Stein 313/340 X 3,204,140 8/1965 Kearns 313/239 Primary Eicaminer]ames W. Lawrence Assistant ExaminerSaxfield Chatmon, Jr.

Attorney, Agent, or FirmW. H. MacAllister, Jr.; R. A. Cardenas '5 Claims, 4 Drawing Figures PRIOR ART 1 INSULATED CATHODE GUN DEVICE FIELD OF THE INVENTION power than previous insulated cathodes.

DESCRIPTION OF THE PRIOR ART A traveling wave tube includes an electron gun assembly and a collector assembly coupled together by a slow-wave structure. The electron gun assembly emits a stream of electrons which travels through the slowwave structure and is collected by the collector assembly. The stream of electrons is caused to interact'with a propagating electromagnetic wave in a manner which amplifies the electromagnetic energy. In order to achieve such interaction, the electromagnetic wave is propagated along the slow-wave structure, such as a conductive helix wound about the path of the electron stream. The slow-wave structure provides a path of propagation for the electromagnetic wave such that the traveling wave effectively propagates at nearly the velocity of the'electron stream.

An electron gun assembly may include a cathode heater, a cathode electrode, a focusing electrode, an

accelerating anode and the required support structure for mounting all these components. .The cathode electrode is usually a tubular member having an electron emissive or dispensing cap commonly called a cathode at one end-The cathode heater may be any convenient shape such as a helix, for example, that fits into the cavity of the cathode electrode. The focusing electrode, disposed between the cathode electrode and the slowwave structure, is also a tubular member to which an electric field may be applied for focusing the electron stream emitted from the cathode electrode. An accelerating anode may be used to accelerate the electrons in the stream before the-electrons enter the slow-wave structure. The above-mentioned electrical components are positioned in a supporting structure which is attached to one end of the slow-wavestructure.

Thermionic emission from the cathode takes place when the cap is heated to approximately 1,000C by the cathode heater. Along with the cathode heater and the cathode being heated to 1,000C the other components and the support structure are also heated to approximately the cathode temperature by both radiation and conduction from the cathode electrode. Since essentially the entire electron gun assembly is heated by the cathode heater to a very high temperature, sufficient power must be supplied to the cathode heater to maintain the cathode at the proper thermionic emission temperature. The requirement of additional power to maintain the entire cathode gun assembly at an elevated temperature may place excessive demands on a limited power source such as in a space vehicle. Also, heating of components other than the cathode heater and the cathode induces problems of alignment of these other electrodes, i.e. metals expand causing position changes of these various electrodes making alignment difficult. High temperature operation may also tend to induce metal fatigue resulting in a shortened life for the cathode gun assembly.

mionic emission. l5

Various prior art solutions for reducing the power requirements of a lWls electron gun assembly have Accordingly, it is an object of the present invention to provide a simple, reliable and more efficient traveling wave tube.

It is another object of the present invention to provide a traveling wave tube electron gun assembly operating at a lower temperature. T

It is a further object of the present invention to provide a cathode electrode requiring less power for ther- It is a still further object of the present invention to provide an insulated cathode electrode that transfers a limited amount of heat to its surroundings.

It is still another object of the present invention to provide an insulated cathode electrode having a shield that is compact and stable under vibration.

. In accordance with the foregoing objects, a cathode gun device according to the invention includes a cathode mounted to one end of a tubular cathode support member. A cathode heater is provided within the tubular cathode support member with terminals extending away from the cathode end, A first composite heat shield, comprising a plurality of concentric layers is disposed about substantially the outer surface of the cathode support member. A first layer of the first composite heat shield is afiber metal having low thermal conductivity. A second layer is a thin polished foil made of a low thermal conductivity, low emissivity metal sheet. A second composite heat shield made of a plurality of flat layers is disposed about the end opposite the cathode end of the cathode support member. An electrical insulating spacer is provided extending through the second heat shield for providing electrical insulation between one heater terminal and the second heat shield. A first layer of the second composite heat shield is a thin polished foil made of a low thermal conductivity, low emissivity metal sheet. A second layer is a fiber metal having low thermal conductivity.

The foregoing and other objects and features of the present invention will become readily-apparent from the following description of preferred embodiments of the invention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS DETAILED DESCRIPTION OF THE DRAWINGS Referring more specifically to FIG. 1, a cathode electrode device according to the prior art may be seen to include a cathode electrode 10, a heater electrode 20, and a first heat reflective shield 30 and'a second heat reflective shield 40. The cathode heater 20 heats the cathode electrode 10 to thermionic emission temperature, and the heat reflective shields 30 and 40 reflect back some heat that is radiated by the cathode electrode 10.

The cathode electrode includes an electron emitting cathode cap 11, a cathode support member 14, and a cathode sleeve 16. The electron emissive cathode cap, hereinafter. called cathode 11, is a cylindrically shaped disk with a concave surface at one end 12 from which electrons are emitted and a flat surface at the other end 13. The cathode 11 may be made of a material suitable for electron emission when heated to the proper thermionic temperature such as molybdenum or tantalum, for example. The flat end 13 of the cathode 11 is mounted to one end of a tubular cathode support member 14 which may be made of a metal such as mo-' lybdenum. The other end 15 of the cathode support member 14 is mounted to a tubular cathode sleeve 16 that is made of a material having a lower thermal conductivity than the cathode support 14 such as tantalum.

A heat reflective shield 30 is disposed about the circumference of the cathode 11, the cathode support member 14, and, is attached to the cathode sleeve 16. The heat reflective shield 30 is'tubular in shape with a substantial portion of its length having an inside diameter larger than the cathode 11 or the cathode support member 14 such that the heat reflective shield 30 is spaced away from cathode l1 and the cathode support 14. One end of the heat reflective shield 30 has a sufficiently reduced'diameter to be attached onto the cathode sleeve 16 by any convenient method suchas spot welding, for example, along the circumference, one point of which is indicated as point 31.

Located within the cathode electrode 10 is the cathode heater 20 for heating the cathode 11 to thermionic emission temperature of approximately 1,000C. The cathode heater 20 is in close proximity to the cathode 11 for maximum heat transfer to the cathode 11. The heater 20 includes a hellical coiled wire forming the heater element 21 and terminals'22 and 23. An insulating structure 24, such as a suitable high temperature potting compound, may be used tospace the cathode heater 20 away from the cathode 11 and the cathode support 14. Terminals 22 and 23 of the cathode heater element 21 may extend beyond the'cathode sleeve 16 for convenient electrical connection through the disk shaped ceramic insulator 25. t

A second heat reflective shield 40 is disposed within the cathode electrode 10 near the region of the end 17. The shield 40 is comprised of three individual heat reflective shields 41 that are 0.0002 inch thick discs having polished surfaces and are made of a low emissivity material such as molybdenum, for example. Each disc has 2 holes, diametrically opposed on the face of the disc, through which tubular spacers 42 may be passed for separating the individual discs from each other and for providing electrical insulation between the shield 40 and the heater terminals 22 and 23.

In operation of the device of FIG. 1, power is applied to the terminals 22 and 23 of the cathode heater 20, thereby heating the cathode 11 to thermionic emission temperature, approximately l,0O0C. The cathode support 14 and the cathode sleeve 16 are also heated-by the cathode heater 20. Upon heating, these structures will radiate a substantial amount of heat, approximately 50 percent, away to their surroundings. With the heat electrode 10. However, the shield will itself be heated as it conducts heat away from the area where the shield 30 is attached to thecathode sleeve 16. The shield 30 will then radiate this heat energy to its surroundings; It is thereforeapparent that even using such a heat shield there is still loss of heat and undue consumption of power due to unnecessary heating of the entire electron gun assembly by the cathode electrode 10. Also, such a heat reflective shield 30 is unstable during period of mechanical vibration. Since one end of the heat shield 30 is unsupported, it is free to oscillate in response to mechanical vibration and may cause mechanical and electrical malfunction of the electron gun assembly by shorting out some of the neighboring electrodes and structure.

Heat is radiated from the cathode heater element 21 toward the end 17 of the cathode electrode 10 and the heat shield 40. The side of the shield 40 facing the heater element 21 reflects some heat back to that element while the other side radiates some heat resulting in heat loss from the cathode electrode 10. Under vibration conditions, the individual shields 41 are relatively free to vibrate since they are not heldrigidly and there is the possibility of electrical shorting of the terminals 22'and 23 by the shields 41. V

Referring more specifically to FIG. 2', a cathode gun device according to the invention may be seen to include a cathode electrode 10 containing a cathode heater 20, both as described in FIG. 1 above, and a composite heat shield disposed about the substantial outer'surface of the cathode electrode 10. The composite heat shield 50 is composed of a plurality-of layers of thermal insulators and reflectors in an alternate arrangement'. The first layer, placed closest to the cathode electrode 10 surface, is a high temperature thermal insulating heat shield 51. The shield 51 may be a sintered metal or a foamedmetal material such as nickel,

reflective shield 30 in place, some of the outwardly radiated thermal energy is reflected back to the cathode for example. Fiber metal can withstand the high temperatures necessary for thermionic emission and hasa relatively low thermal conductivity. A metal in the form of relatively long fibers having a relatively small corss-sectional area and a relatively long mean free path between adjoining fibers, will have relatively small heat conduction. Mean free path between fibers refers to the distance along a given fiber between contact to other fibers.

The following equation maybe seen to describe the steady state thermal transfer between the ends ofa fiber due to conduction:

Q K A /L AT Where Q heat in calories/sec; K thermal conductivity of the material in calcm/cm -C-'sec;

A cross-sectional area a 'flber'to the heat flow in cm L mean free path or length of the fiber in cm; and

AT= temperature difference between the fiber ends.

Assuming a predetermined temperature difference between the ends of a fiber, it can be seen from the equation that there is minimum heat flow between the ends when there is a minimum K and a minimum A /Lratio. A minimum A IL ratio is obtained by minimizing the cross-sectional area A and maximizing the length L. A, is minimized when there is a small diameter fiber,

such as 0.000015 inch, for example, and length L is maximized when the mean free path is long relative to the diameter. Therefore, the ratio A /L is small. Minimum heat flow along the length of the foil is also provided by using materials having a low thermal conductivity constant K. A sintered fiber metal marketed by Huyck Metals Company under the tradename FELT- METAL has been found as a satisfactory insulator.

It is again pointed out that there are several insulation materials that may be used in practicing the invention, including, for example, a foamed metal and the invention is not limited to the above-described fiber metal.

The second layer, Wrapped about the first layer 51, is a heat reflective shield 52 having a polished'inside surface 52A for reflecting any heat that is incident to that surface. The heat reflective shield 52 may be a thin sheet metal foil made of low thermal conductivity and low emissivity material such as molybdenum, for example. The foil having a thickness of about-0.0002 inch is preferred for its relatively low mass and the resulting low heat capacity. A heat shield with a polished inner surface 52A will reflect a substantial portion of radiant heat incident to'that surface andif it has low emissivity, a small amount of heat will be radiated by the outer surface 52B. r

A third layer may be wrapped around the heat reflective shield 52, as described above, for further thermal insulation. The third layer is a thermal insulator heat Referring now to FIG. 4, another embodiment according to the present invention includes a cathode electrode 10, a heater electrode 20, encapsulated in an insulating structure 24, and a heat shield 60. The cathode electrode 10, the heater electrode 20 and the insulating structure 24 around the heater element 21 are similar to the items in FIGS. 1 and 2 having-the same reference designations. Heater terminals 22 and 23 extend outward from the heater element 21 through the ceramic insulator disk 25, and beyond the end 17 of the cathode electrode forv making an electrical connec tion. Tubular electrical insulating spacers 70 having a shouldered end 71 are slipped over the terminals 22 and 23 with the shouldered end facing away from the heater element 21. The spacer 70 may be made of any suitable high temperature material that is capable of I withstanding temperatures in excess of 1,000C such as shield 51 similar to the first layer'heat shield51 described above.

A' fourth layer may be wrapped about the third layer forming a heat reflective shield 52, similar to the second layer heat shield 52 described above, for providing additional insulation.

It is pointed out that a composite heat shield 50 may be composed of as many individual-heat shields 51 and 52 as may be required or as there is room for in the cathode gun assembly. The composite heat shield 50 may be considered as being made up of at least one 51 and one 52 shield. The heat shields 51 and 52 may be interchanged with each other and they need not be in the order described.

The composite heat shield 50 may be fastened to the cathode electrode 10 by any one of a number of methods. The individual shields 51 and 52 may be attached one at a time by spot welding or by placing a tubular sleeve 53, shown in dashed lines, over the outer layer. Another convenient method to fasten the heat shield 50 is to assemble the individual shields-51 and 52 in a flat pattern and then sinter the layers together in a vacuum at a temperature between 900 and l,000C. The sintered composite heat shield 50 may then be wrapped about the cathode electrode 10 and spot welded in place.

Referring now to FIG. 3, the top view of the invention according to FIG. 2 is illustrated in FIG. 3. The cathode electrode 10 is seen surrounded by the heat shield 50 which is composed of the individual shields 51 and 52. The heat shield 50 is attached to the cathode electrode 10 by spot welding along the line extending downward (into the drawing) from the points 541 and 55. An alternative method of mounting the heat shield 50 is by placing a sleeve 53 shown in the dashed line, around the shield 50, thereby holding the shield 50 in place by friction.

a ceramic, for example. A disk-like composite heat shield 60 having two diametrically opposed holes 67 extending from face to face of the disk is placed over the two electrical insulating spacers 70. The shoulder 71 on the spacer 70 is larger than the diameters of the holes 67 in the heat shield 60 so that the shoulder 71 rests against the face of the heat shield 60 that faces the heater element 21. Tubular electrical insulating spacers 75 having larger outside diameters than the diameters of the holes 61 in the heat shield are slipped over the heater terminals 22 and 23 and rest against the back face of the heat shield 60. The spacer 75 may be made of the same material as the spacer 70. The heat shield and the spacers and are held in place on the heater terminals 22 and 23 by retainer bars 66 welded at the shouldered endof spacers 70 and next to the spacers 75. v

The composite heat shield 60 is composed of a plurality of layers of thermal insulators and reflectors in an alternate arrangement similar to the arrangement of the heat shield 50. The first layer, closesttothe heater element 21, is a heat reflective shield 61 which is made of the same material as the heat reflective shield 52 described in FIG. 2 above. The second layer is an insulating shield 62 that is made of the same material as the shield 51 described in FIG. 2 above. The third layer is another heat reflective shield 61, the fourth layer is a thermal insulating shield 62, and the fifth layer is a heat reflective shield 61. It is pointed out that additional shields 61 and 62 may be used to improve the insulating qualities of the heat shield 60. The combination of at least one shield 61 and one shield 62 may be considered as an elemental heat shield 60.

The operation of the invention will be described in relation to the embodiments of FIGS. 2, 3, and 4. Power is applied to the cathode heater terminals 22 and 23 causing the'cathode heater to attain a thermionic emission temperature of approximately l,000C. The cathode heater 20 heats the entire cathode electrode 10 and its supporting structure to about the same temperature. The exterior surfacesof the cathode electrode 10 will radiate heat to its surroundings and the composite heat shield 50 will substantially reduce the amount of heat that is radiated away. The first layer of the composite heat shield 50 is the thermal insulating shield 51 which has a low thermal conductivity and therefore conducts only a small amount of heat to the second layer, i.e., the thermal reflectiveshield 52. A substantial portion of the radiant'heat fallingon the inside surface of the heat reflective shield 52 will be reflected back to the cathode electrode 10. The reflective shield 52 also being of low thermal conductivity will conduct only a small amount of heat from its inside sur' face to its outside surface. The third and fourth layers of the heat shield 50 function substantially as described for the first and second layers 51 and 52.

One of the advantages of a heat shield 50 as described above'is that during periods of high mechanical vibration the shield is not free to vibrate. Since the heat shield 50 is mounted directly to the cathode electrode 10 along its entire length, it is sufficiently supported to prevent it from vibrating and causing any short circuits while providing substantial thermal insulation.

The cathode heater 20 will radiate heat axially to the cathode electrode 10 and the composite heat shield 60 will substantially reduce the amount of heat being radiated from the end 17 of the cathode electrode 10. The operation of the heat shield 60 is substantially similar to the operation of the heat shield 50 as described above. Since the heatreflective'shields 61 are separated by the rigid thermal insulating layer 62, they are not free to vibrate and the possibility of the heater terminals 22 and 23 being. electrically shorted to each other is substantially eliminated. c

It should be apparent from the. foregoing that the present invention provides a simple and reliable insulated cathode electrode. Moreover, the device may reduce the input power required to maintain the cathode v at thermionic emission temperature by a substantial amount. In fact, a cathode electrode according to FIGS. 2 and 4 has been constructed andtested and it was found to have an increased operating efficiency of percent.

Although the present invention has been shown and described with reference to particular embodiments, nevertheless, various changes and modifications obvious to one skilled in the art to which the invention pertains are. deemed to lie within the purview of the invention.

I claim:

1. A cathode gun device comprising:

a cathode for being heated to thermionic emission temperature;

a tubular cathode support member having said cathode mounted on one end; i r

a heater electrode being mounted within said cathode support member;

a first heat shield having a heat reflective surface and having a low emissivity being mountedabout the end oppositethe cathode;

a second heat shield havinga low thermal conductivity mounted next to said first heat shield;

a third heat shield substantiallysimilar to said first heat shield being mounted next to said second heat shield; and

means for securing said heat shields to each other and to said heater electrode.

2. A cathode gun device according to claim 1 wherein: said second'heat shield is a fiber metal material. 1 v

3. A cathode gundevice comprising:

a cathode for heating to thermionic emission temperature; r

' a tubular cathode support member having said cath- 4. A cathode gun device comprising: i

a cathode for heating to thermionic emission temperature;

a tubular cathodesupport member having said cathode mountedon one end;

a heater electrode being mounted within said cathode support member;

a first heat shield having a heat reflective surface and having a low emissivity being mountedabout said cathode support member;

a second heatshield having a low thermal conductivity mounted next to said first heat shield;

a third heat shield substantially similar to said first heat shield being mounted next to said second heat shield; and

means for securing said heat shields to each other and to said cathode support member.

5. A cathode gun device according to claim 4 wherein: said second heat shield is a fiber metal material.

i III I l 

1. A cathode gun device comprising: a cathode for being heated to thermionic emission temperature; a tubular cathode support member having said cathode mounted on one end; a heater electrode being mounted within said cathode support member; a first heat shield having a heat reflective surface and having a low emissivity being mounted about the end opposite the cathode; a second heat shield having a low thermal conductivity mounted next to said first heat shield; a third heat shield substantially similar to said first heat shield being mounted next to said second heat shield; and means for securing said heat shields to each other and to said heater electrode.
 2. A cathode gun device according to claim 1 wherein: said second heat shieLd is a fiber metal material.
 3. A cathode gun device comprising: a cathode for heating to thermionic emission temperature; a tubular cathode support member having said cathode mounted on one end; a heater electrode being mounted within said cathode support member; a first heat shield having a heat reflective surface and having a low emissivity being mounted about said cathode support member; a second heat shield being fiber metal and having a low thermal conductivity mounted next to said first heat shield; and means for securing said heat shields directly to each other and directly to said cathode support member.
 4. A cathode gun device comprising: a cathode for heating to thermionic emission temperature; a tubular cathode support member having said cathode mounted on one end; a heater electrode being mounted within said cathode support member; a first heat shield having a heat reflective surface and having a low emissivity being mounted about said cathode support member; a second heat shield having a low thermal conductivity mounted next to said first heat shield; a third heat shield substantially similar to said first heat shield being mounted next to said second heat shield; and means for securing said heat shields to each other and to said cathode support member.
 5. A cathode gun device according to claim 4 wherein: said second heat shield is a fiber metal material. 